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Equipment Discussions >> Eyepieces

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Sarkikos
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Reged: 12/18/07

Loc: Nyctophobia, Maryland, USA
XW 20 in a 10" f/4.8 Dob
      #5608745 - 01/06/13 04:18 PM

Recently I bought an XW 20mm eyepiece at a good price from a good friend. (Thanks!) I had read that the 20mm focal length in the XW series has terrible field curvature (FC). This is the aberration that causes stars to appear out of focus toward the edge of field while they are sharply focused on-axis. If you sharply focus the stars near the edge of field, they will be out-of-focus on-axis and vice versa. But this XW 20 was offered to me at a price I could not refuse, so I didn't refuse it!

Now there are a few factors which will affect how the FC in an eyepiece will appear to a particular observer. One is age. Many younger people are able to focus their eyes to compensate for FC. They probably won't even realize they are doing this, and will be amazed that others see FC in an eyepiece in which they see none themselves. There is no use arguing about this difference. It's just the way it is. Usually people tend to lose much of the focusing range of their eyes as they grow older. I'm one of these people. My first experience with obvious and aggravating FC was in a pair of ES 82 degree 14mm eyepieces I had intended for binoviewing. I could not deal with having to refocus the image of Jupiter as it drifted across the FOV. I eventually sold the pair.

But what is a problem for eyepieces when binoviewing planets in suburbia might not be so bad when viewing DSO at a dark site. I intended to find out. So I took the XW 20 along with me when I recently went for an observing session with my 10" f/4.8 Dob to bag some more H400 objects.

I usually have the Baader Zoom in my focuser when working on DSO lists. I can definitely get more done if I don't have to switch among eyepieces hunting for the optimum magnification. But this night was not starting out so well. The entire sky was patchy with clouds. I decided to give the XW 20 a try while I waited for the sky to clear.

I began by viewing objects and areas which would show me a large swathe of stars across the entire FOV in order to check out the FC. I looked at M45, Cr 70 (Orion's Belt), and open cluster NGC 1981 north of M42. Yes, indeed the FC in the XW 20 was obvious to my eyes! I no longer had an ES 82 14 to compare with it, but between the two, the XW 20 quite possibly had even worse FC. If I focused on-axis, the stars at least half-way to the edge-of-field were already strongly out-of-focus. The stars near the EOF were terrible. And there was no use trying the old trick of focusing on stars half-way to EOF, or two-thirds to EOF or whatever position some experienced observers advise. I was not able to play with the focus in this way to come to an acceptable level of FC across the FOV. It just wasn't happening!

But luckily I have a Paracorr Type I. I had heard - and I'ved seen for myself - that a Paracorr can help flatten the field of eyepieces. So I primed the Paracorr with my TV Plossl 25 at setting 4, locked the focuser, and put in the XW 20. Turning the tuning top, I found out that the Paracorr Type I does not have enough in-focus to accommodate the XW 20. The next best setting was all the way in, Setting 5. At this setting the XW 20 was a bit out of focus. I unlocked the focuser and turned the focuser knob a little.

Now when I focused for stars on-axis, virtually all the stars across the FOV in the XW 20 were in focus! Only the stars toward the extreme edge were a bit fuzzy. But when I tried the trick of focusing for stars between on-axis and EOF, the entire FOV was acceptably sharp and the field was flat. Color rendition was excellent and transmission appeared high. The view was so good, that I left the XW 20 and Paracorr in the focuser for 90% of the rest of the night! (That honor is usually reserved for my Baader Zoom.)

Well, soon a strong west wind started blowing all those clouds away. The sky was transparent at least until the Moon appeared over the trees at about 1 AM. With the XW 20 I was able to bag 35 more objects - mostly galaxies - from the H400's. This was a record for me. Not bad for a scope with no goto or DSCs. The only time I took out the XW 20 was to try some eyepieces around 25mm to finally catch sight of the Horsehead Nebula (B33) in Orion.

So my verdict is that for myself and others who cannot focus their eyes to compensate for FC, the XW 20m is a great eyepiece for DSO hunting in an f/5 Dob ... as long as it is kept in a Paracorr. I imagine the view would be even better in a Paracorr Type II which has enough in-focus to accommodate the XW 20. But even in a Type I this eyepiece did a great job. I believe the XW 20 will remain in my deep sky case for the foreseeable future. Or at least until I find a Delos at a price I can't refuse!

Mike


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GeneT
Ely Kid
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Reged: 11/07/08

Loc: South Texas
Re: XW 20 in a 10" f/4.8 Dob new [Re: Sarkikos]
      #5609009 - 01/06/13 06:40 PM

Quote:

the XW 20m is a great eyepiece for DSO hunting in an f/5 Dob ... as long as it is kept in a Paracorr.




My 22 Nagler performs quite well in my 12.5 inch F5 Dob--without a Paracorr. Now, the edge of the field does show coma. If you want good views to the edge of the field, you do need to use a Paracorr.


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JayinUT
I'm not Sleepy
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Reged: 09/19/08

Loc: Utah
Re: XW 20 in a 10" f/4.8 Dob new [Re: GeneT]
      #5609056 - 01/06/13 07:10 PM

Mike,

Excellent post. Thanks for sharing your findings.


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Starman81
Carpal Tunnel
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Reged: 03/06/08

Loc: Metro Detroit, MI, USA
Re: XW 20 in a 10" f/4.8 Dob new [Re: JayinUT]
      #5609080 - 01/06/13 07:20 PM

Nice report Mike, thanks. Looks like I won't be binoviewing with your XW 20 after all! One day, I plan to get a larger scope and might need a Paracorr, so I am glad to see that it may have the additional benefit of cleaning up the views in the longer FL XWs.

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BillP
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Reged: 11/26/06

Loc: Vienna, VA
Re: XW 20 in a 10" f/4.8 Dob new [Re: Sarkikos]
      #5609109 - 01/06/13 07:36 PM

I love my 20XW in all my scopes!

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Sarkikos
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Reged: 12/18/07

Loc: Nyctophobia, Maryland, USA
Re: XW 20 in a 10" f/4.8 Dob new [Re: BillP]
      #5609196 - 01/06/13 08:35 PM

Bill,

Do you love the XW 20 in your 10" f/5 Dob without a Paracorr?


Mike


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johnnyha
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Reged: 11/12/06

Loc: Sherman Oaks, CA
Re: XW 20 in a 10" f/4.8 Dob new [Re: Sarkikos]
      #5609306 - 01/06/13 09:55 PM

I had a similar experience the last few nights working with my Leica ASPH zoom and the Paracorr 1 in my f4.65 dob. The naked Leica started getting mushy 2/3 of the way to the edge but the Paracorr 1 cleaned it right up and gave me a nice sharp image of Jupiter across the FOV. I was extremely impressed.

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Scanning4Comets
Markus
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Reged: 12/26/04

Loc: Ontario, Canada
Re: XW 20 in a 10" f/4.8 Dob new [Re: johnnyha]
      #5609361 - 01/06/13 10:26 PM

Mike, Good post!

I have tried JunoMike's 20mm XW in my 10" F/4.7 w/o a Paracorr, and I will say that I *can* see very minimal FC and that it is not distracting in any way. I just turned 50 on Dec 15th, so my older eyes are not affected like some with this particular eyepiece. I was extremely close to buying one until I tried Mike's 22mm Vixen LVW, (Thanks Mike!), and I was also lucky enough to try one out before I bought it off him and glad I did because stars are nice little round pinpoints w/o a Paracorr in my scope.

That's not to say I don't like the 20mm XW because I do ~ I just went the other route because I had the $$$ at the time and I saved myself $100.00 LOL!

I find I get that "immersion factor" with the 20mm XW more than the 22mm LVW, but both are really nice EP's. IMO, I could prob skip my 22m LVW and go right from my 34mm ES to the 14mm Denk, (35x to 86x).

Cheers,


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BillP
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Reged: 11/26/06

Loc: Vienna, VA
Re: XW 20 in a 10" f/4.8 Dob new [Re: Sarkikos]
      #5609471 - 01/06/13 11:38 PM

Quote:

Bill,

Do you love the XW 20 in your 10" f/5 Dob without a Paracorr?


Mike




If you look at the 24-26mm Review I did, you will note that just about all the eyepieces showed the XT10's coma at the same point in their FOV, or about 1/2 way. So the question really should be if I like ANY 20mm-ish eyepiece in my XT10 w/o Paracorr? Answer is yes When using a long focal length eyepiece in my XT10 w/o Paracorr, I know what the limitations will be an just accept them. The outer half of the FOV is going to be aberrated. So I just focus on the central part of the FOV and leave the off-axis to my less critical peripheral vision. Doing this things look just fine and can focus on the very engaging view the XW provides, along with its very comfortable exit pupil.

I used to have the 24 Pan as my largest TFOV 1.25" eyepiece, but after getting the XW, even with it's FC, the more important (for me) on-axis was more engaging and had a lot more "pop" to the star fields. So I traded off the off-axis for a better on-axis and sold the 24 Pan. If some day Tele Vue decides to replace the 24 Pan with another 24mm-ish max TFOV 1.25" eyepiece, and gives it a nice 20mm of eye relief with their reputation for a superb off-axis, then I will get it. But until that day comes, to my eye the 20XW wins over the 24 Pan. Then when I move to my TSA-102, if becomes even less of a contest. The XW line is really a great eyepiece line. Sure it's got some faults, but they all do someplace. What I enjoy is its strengths...in my XT10, or other scopes.


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Sarkikos
Postmaster
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Reged: 12/18/07

Loc: Nyctophobia, Maryland, USA
Re: XW 20 in a 10" f/4.8 Dob new [Re: johnnyha]
      #5609775 - 01/07/13 08:59 AM

Johnny,

Quote:

I had a similar experience the last few nights working with my Leica ASPH zoom and the Paracorr 1 in my f4.65 dob. The naked Leica started getting mushy 2/3 of the way to the edge but the Paracorr 1 cleaned it right up and gave me a nice sharp image of Jupiter across the FOV. I was extremely impressed.




I've been thinking about getting a Leica ASPH, but was leary about paying so much for an eyepiece that will not do so well in a fast Dob. Of course, I'd have to sell most of my other eyepieces to afford it, but others have already done so! Sounds like the man who sold all his possessions to enter the Kingdom of Heaven.

The fact that the Leica cleans up well in a Paracorr is good to hear. I wouldn't have to Barlow it just to get a better field of view.

Mike


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Sarkikos
Postmaster
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Reged: 12/18/07

Loc: Nyctophobia, Maryland, USA
Re: XW 20 in a 10" f/4.8 Dob new [Re: BillP]
      #5609798 - 01/07/13 09:14 AM

Bill,

Quote:

If you look at the 24-26mm Review I did, you will note that just about all the eyepieces showed the XT10's coma at the same point in their FOV, or about 1/2 way. So the question really should be if I like ANY 20mm-ish eyepiece in my XT10 w/o Paracorr? Answer is yes




Well, not exactly. My concern wasn't coma at all. To my eyes the most obvious aberration in the XW 20mm when used in my 10" f/4.8 Dob is field curvature. Since a Newt with a 1200mm focal length has an essentially flat field, the FC is entirely due to the eyepiece.

I know that FC was what I was seeing in the XW 20 because if I focused sharply on-axis, the off-axis would be obviously out of focus, becoming worse toward the EOF, and vice versa. The old trick of focusing midway between on-axis and EOF did not alleviate the FC, but just made it appear both on-axis and toward EOF. Not too good. I used the Paracorr in the hopes of flattening the field, which it did very well. For the XW 20, any coma correction was just icing on the cake.

Mike


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Sarkikos
Postmaster
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Reged: 12/18/07

Loc: Nyctophobia, Maryland, USA
Re: XW 20 in a 10" f/4.8 Dob new [Re: Sarkikos]
      #5609799 - 01/07/13 09:15 AM

Bill,

Quote:

When using a long focal length eyepiece in my XT10 w/o Paracorr, I know what the limitations will be an just accept them. The outer half of the FOV is going to be aberrated. So I just focus on the central part of the FOV and leave the off-axis to my less critical peripheral vision. Doing this things look just fine and can focus on the very engaging view the XW provides, along with its very comfortable exit pupil.




Yes, often I use long focal length eyepieces without bothering to insert them in my Paracorr. The image appears better with the coma cleaned up, but I can live with it for a quick look.

But I cannot live with obvious FC. I just won't do it. That's why I sold my pair of ES 82 14mm. I couldn't use the Paracorr with my binoviewer to help flatten the field in those, so out they went.

Mike


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Sarkikos
Postmaster
*****

Reged: 12/18/07

Loc: Nyctophobia, Maryland, USA
Re: XW 20 in a 10" f/4.8 Dob new [Re: BillP]
      #5609809 - 01/07/13 09:25 AM

Bill,

Quote:

I used to have the 24 Pan as my largest TFOV 1.25" eyepiece, but after getting the XW, even with it's FC, the more important (for me) on-axis was more engaging and had a lot more "pop" to the star fields. So I traded off the off-axis for a better on-axis and sold the 24 Pan.




Yep, the 24 Pan is one of those eyepieces that I've heard so much praise about but I've always resisted buying because of the price. Even used, they cost more than I want to pay for them. For what I want out of an eyepiece - high transmission, sharper on-axis, but with a decent FOV - the XW's just made more sense to me.

Quote:

If some day Tele Vue decides to replace the 24 Pan with another 24mm-ish max TFOV 1.25" eyepiece, and gives it a nice 20mm of eye relief with their reputation for a superb off-axis, then I will get it. But until that day comes, to my eye the 20XW wins over the 24 Pan.




My Meade 5k SWA 24 fills the niche that a 24 Pan would occupy in my deep sky case. I don't feel the need to replace it with a Pan.

Mike


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BillP
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Reged: 11/26/06

Loc: Vienna, VA
Re: XW 20 in a 10" f/4.8 Dob new [Re: Sarkikos]
      #5609820 - 01/07/13 09:36 AM

Quote:

Bill,

Quote:

When using a long focal length eyepiece in my XT10 w/o Paracorr, I know what the limitations will be an just accept them. The outer half of the FOV is going to be aberrated. So I just focus on the central part of the FOV and leave the off-axis to my less critical peripheral vision. Doing this things look just fine and can focus on the very engaging view the XW provides, along with its very comfortable exit pupil.




Yes, often I use long focal length eyepieces without bothering to insert them in my Paracorr. The image appears better with the coma cleaned up, but I can live with it for a quick look.

But I cannot live with obvious FC. I just won't do it.




We are all different in our likes or dislikes. Why no one eyepiece can ever be best. In a Newt w/o Paracor, coma is an aberration I can't fix, so the off-axis is permanently mucked up. With FC, if I am lazy and don't re-center my area of interest, a small turn of the focuser know makes it nice and clear. I don't consider the FC all that bad in the 20XW...and its so little in the 14XW that my eyes can accomodate it. The 14XW is actually my favorite eyepice of all the XWs.


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Sarkikos
Postmaster
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Reged: 12/18/07

Loc: Nyctophobia, Maryland, USA
Re: XW 20 in a 10" f/4.8 Dob new [Re: BillP]
      #5609847 - 01/07/13 10:04 AM

Bill,

Apparently my eyes have lost much of their ability to focus. I need the focuser to do that for me, and I consider it a royal PITA to have to fiddle with the focuser when going from on-axis to half-way-to-EOF to near-EOF. Just not worth it for me.

FWIW, my 56-year-old eyes seem to have retained their ability to dilate widely in the dark. My irises still dilate to about 7mm. Of course, this is only useful for naked eye and at very low power through the telescope. Also, the sharpness for fine planet-surface detail seems to be there.

I guess I should be thankful for what I have and avoid eyepieces with FC ... unless I put them in the Paracorr.

Mike


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coutleef
Post Laureate
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Reged: 02/21/08

Loc: Saint-Donat, Quebec, Canada
Re: XW 20 in a 10" f/4.8 Dob new [Re: Sarkikos]
      #5610439 - 01/07/13 03:30 PM

dear Mike

thanks for your report. it has been my experience as well. since i got pentax XW from 40 down to 7 mm, i just do not feel any urge to buy new EPs, except for the baader zoom!! but the zoom makes mu life easier when it is -20!!


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Scanning4Comets
Markus
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Reged: 12/26/04

Loc: Ontario, Canada
Re: XW 20 in a 10" f/4.8 Dob new [Re: coutleef]
      #5610456 - 01/07/13 03:40 PM

You know something...I never gave that any thought...

Quote:

Dear Mike,

Thanks for your report. It has been my experience as well. since I got pentax XW from 40 down to 7 mm, I just do not feel any urge to buy new EPs, except for the baader zoom!! but the zoom makes my life easier when it is -20!!




...I was out this morning and it was pretty cold after an hour. A Zoom eyepiece makes so much sense in the cold!!!

Just one eyepiece and you can see a lot without having to change eyepieces! Great idea! That will be next on my eyepiece list! Looks like I will have to get the Baader Zoom then!

Cheers,


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Starman81
Carpal Tunnel
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Reged: 03/06/08

Loc: Metro Detroit, MI, USA
Re: XW 20 in a 10" f/4.8 Dob new [Re: Scanning4Comets]
      #5610526 - 01/07/13 04:22 PM

I had the Nexstar 4SE out on the AT Voyager mount a few nights ago and all I grabbed was the Baader Zoom and the XW 20. You WILL appreciate having a nice, wide AFOV finder eyepiece on hand to complement the Zoom.

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Sarkikos
Postmaster
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Reged: 12/18/07

Loc: Nyctophobia, Maryland, USA
Re: XW 20 in a 10" f/4.8 Dob new [Re: Starman81]
      #5611915 - 01/08/13 01:07 PM

What I missed when I was using the XW 20 was the ability to dial in the optimum magnification and contrast to bag the very faint fuzzies or to see structure in the brighter ones. But the Baader Zoom and XW 20 together make a good team!

Mike


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Starman1
Vendor (EyepiecesEtc.com)
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Reged: 06/24/03

Loc: Los Angeles
Re: XW 20 in a 10" f/4.8 Dob new [Re: Sarkikos]
      #5612220 - 01/08/13 04:01 PM

The 20XW and 14XW both have more field curvature than other focal lengths, but both seem just fine if a Paracorr is used, at least in my f/5 scope.

The 22 Nagler, which I owned for many years (I liked it enough only the 21 Ethos bumped it from my kit), has a 31.1mm field stop. The coma-free field (where coma is smaller than the Airy disc) at f/5 is, depending on standards, 2.2 to 2.75mm, so only the center spot of the eyepiece is coma free.

At 15mm off-axis, the comatic star image at f.5 is about 0.03mm across.
Tha Airy disc is about 0.005mm, so the star image is literally 6X as wide as the Airy disc 15mm off axis.

Would this be visible? Yeah. Especially since, unlike defocus from field curvature, it extends the star image radially. I thought the 22 Nagler was nearly unusable without coma correction. But then, I bought it before I got a coma corrector, so I saw first-hand what happened to the star images in the field when a coma corrector was added.

But, if the 22mm results in a very low power in your scope, and astigmatism from your eye dominates at the fairly large exit pupil it might provide in your scope, then coma isn't going to be the most annoying optical issue to solve.

I've used the Baader Zoom, by the way, in a Paracorr, and it's cleaned up quite a bit, too, despite its narrower apparent field.


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